Roof Flashing: How Every Detail Goes On
16 step by step guides
Shingles and metal panels keep the rain off the open field of a roof. Flashing keeps it out everywhere else: where the roof meets a wall, a chimney, a pipe, a skylight, a dormer or a change in pitch. Most roof leaks are not the roofing material failing. They are a flashing that was left out, lapped backwards, nailed in the wrong place or replaced with a tube of caulking.
Every flashing detail follows one rule: the upper layer always laps over the lower one, so water is carried out onto the roof surface and never behind it. Start with the lapping rule and the map of flashing types, then work through the detail you need, from step flashing along a sidewall to the kickout that protects the bottom of it.
In the Fraser Valley, where rain is driven sideways against walls for months at a time, flashing is where a roof earns its lifespan. These guides show how each detail is built properly, what goes wrong, and when it is time to have a roofer look at it.
Flashing Details Guides
- Guide 1Roof Flashing Types and Where Each GoesRoof flashing is thin metal or formed membrane that carries water over any joint in the roof. The main types are step flashing along sidewalls, apron flashing at headwalls, counter flashing over masonry, kickouts at wall ends, valley metal, drip edge at eaves and rakes, pipe and vent flashings, and curb or flange flashings at chimneys and skylights. Each laps over the layer below it so water is shed, not sealed out.Intermediate30 minutes to read
- Guide 2The Lapping Rule: How Every Roof Layer Sheds WaterThe lapping rule says every layer on a roof must lie over the layer below it, like fish scales, so water running downhill always passes over the top of each joint and never behind it. Install roofing from the bottom up, and flashing from the bottom up, so each upper piece laps over the lower. A reverse lap, even a small one, is a leak path that sealant only hides temporarily.Intermediate20 minutes to read
- Guide 3How to Install Step FlashingStep flashing is a row of small L-shaped metal pieces, one per shingle course, woven into the shingles where a sloped roof runs along a wall. Each piece sits on top of a shingle, gets one nail in its upper roof corner, and is covered by the next course. The wall leg runs up behind the siding or wall paper. Every piece laps the one below it by at least 3 inches (75 mm), the minimum head lap in the BC Building Code.Advanced1 to 3 hours per wall
- Guide 4How to Install Counter FlashingCounter flashing is a separate piece of metal that covers the top edge of base flashing (step, apron or valley metal) and is fastened to the wall, not the roof. It is installed in a cut mortar joint called a reglet, on the surface of the wall with sealant, or behind cladding in a two-piece receiver. It should overlap the base flashing by at least 4 inches and hang free.Advanced2 to 6 hours
- Guide 5How to Install Kickout FlashingA kickout flashing is an angled piece of metal installed at the bottom end of a sidewall, where the roof edge meets the wall. It catches the water coming down the step flashing and diverts it into the gutter, not behind the siding. Install it over the drip edge and starter, tight to the wall, with a rise of at least 4 inches and about a 110 degree kick away from the wall.Advanced1 to 2 hours
- Guide 6How to Flash a Headwall (Apron Flashing)A headwall is where the top of a roof slope ends against a wall that rises above it. It is flashed with apron flashing: a continuous L-shaped piece with a roof leg at least 4 inches wide lapped over the shingles, and a wall leg at least 4 inches tall behind the cladding. Install ice and water shield first, fasten only the roof leg, and lap the wall barrier over the top of the apron.Advanced1 to 3 hours
- Guide 7How to Flash a Roof to a Wall by Cladding TypeRoof to wall flashing is the same on every wall, but the cladding changes how it is covered. On vinyl and fibre cement, the wall leg goes behind the siding with the barrier lapped over it and a clearance of about 1/2 to 2 inches above the roof. On stucco, a receiver or weep detail is needed. On brick and stone, counter flashing goes into a reglet over the base flashing.Advanced2 to 6 hours per wall
- Guide 8How to Cut a Reglet for Counter FlashingA reglet is a slot cut into a horizontal mortar joint to receive the top edge of counter flashing. Mark a level or stepped line, cut the joint with a dust-shrouded grinder to at least 1 inch deep, clean it, set the flashing lip into the slot, wedge it every 16 to 24 inches, and fill with a flexible sealant. Silica dust control and a respirator are mandatory.Advanced2 to 5 hours
- Guide 9How to Install a Plumbing Vent Pipe BootA plumbing vent pipe boot is a flange with a rubber collar that seals around a vent pipe where it passes through a shingle roof. Match the boot to the pipe diameter (commonly 2, 3 or 4 inches), slide the upper half of the flange under the shingles above, leave the lower half over the shingles below, and nail only the upper corners. Cracked collars are the usual failure, and sealant is not a repair.Intermediate30 to 60 minutes
- Guide 10How to Install Lead Pipe FlashingLead pipe flashing is a pre-formed lead sheet boot with a tapered collar that is folded down into a plumbing vent pipe, instead of clamping around it. Slide the flange under the shingles above and over the shingles below, nail only the upper corners, then form the collar down inside the pipe with a wood dowel. Wash hands after handling lead and keep it out of rainwater drainage where possible.Advanced45 to 90 minutes
- Guide 11How to Flash a B-Vent or Metal Chimney PipeA B-vent or factory-built metal chimney passes through the roof inside a clearance-rated flashing made for that pipe. Use the flashing supplied or listed by the vent manufacturer, shingle the base over the shingles below and under the ones above, nail only the upper flange, then fit a storm collar over the pipe and seal it with high-temperature sealant. Never alter the vent or its clearance to combustibles.Advanced1 to 2 hours
- Guide 12How to Flash a DormerA dormer is flashed in sections. The front wall gets an apron flashing lapped over the shingles below. Each cheek wall gets step flashing, one piece per course, ending in a kickout at the bottom. Behind the dormer, the dormer roof meets the main roof in valleys, or a headwall if it is a shed dormer. Ice and water shield goes under every joint, and the wall barrier laps over all the metal.Advanced4 to 8 hours
- Guide 13How to Flash a Roof Pitch ChangeA pitch change is where one roof slope meets another at a different angle, such as a steep roof running into a low-slope porch roof or a mansard lower slope meeting a flatter upper one. Lap underlayment and ice and water shield across the break, add a transition flashing or a wide metal drip edge at the lower slope, and confirm the lower slope meets the roofing product's minimum pitch. Low slopes often need membrane roofing.Advanced2 to 6 hours
- Guide 14How to Flash a Roof Mount (Satellite, Solar, Antenna)A roof mount, such as a satellite dish, antenna or solar rack foot, is fastened to the roof with lag screws that must reach a rafter. Use a flashed mount or a separate flashing plate, shingle its upper half under the course above and its lower half over the course below, drive lags through it into the rafter with sealant under the flashing and lag head, and never rely on sealant alone around a bare bolt.Advanced30 to 90 minutes per mount
- Guide 15How to Flash an Electrical Service MastAn electrical service mast is a steel conduit that carries the service wires up through the roof to the weatherhead. The roofer flashes the roof opening with a mast flashing, shingled in like a pipe flashing, with a storm collar or sealed boot. The mast, weatherhead, meter base and any movement of the mast belong to a licensed electrician, and the service drop wires belong to the utility. Never touch or move them.Professional1 to 2 hours for the roofing work, plus scheduling
- Guide 16How to Choose and Apply Roofing SealantsRoofing sealant is a flexible material used at laps, exposed fasteners and penetrations as a backup to lapped flashing. Polyurethane and hybrid polymer sealants suit metal, masonry and exposed joints, butyl tape suits metal laps, and asphalt roof cement suits asphalt shingle details. Clean and dry the surface, use the right bead size, tool it smooth, and never use sealant to replace a missing lap or flashing.Intermediate30 to 60 minutes per detail
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Flashing Details: Common Questions
What is roof flashing?
Roof flashing is thin metal, usually galvanized or prefinished steel, aluminum or copper, installed wherever the roof surface is interrupted: along walls, around chimneys, pipes and skylights, in valleys and at the edges. It is lapped with the shingles or panels so water running down the roof is carried over every joint instead of into it.
What is the most common roof flashing leak?
Roof to wall joints are the most common, especially a missing kickout flashing at the bottom of a sidewall and step flashing that was reused or nailed through the wall leg. Chimney counter flashing and cracked plumbing vent boots come close behind. All of them can leak for a long time inside a wall before a stain appears on a ceiling.
Can flashing be fixed with caulking?
Only as a short term patch. Sealant cracks and pulls away after a few seasons of sun, rain and cold, and a caulked joint has no lap to fall back on when it fails. A lasting repair replaces or corrects the metal so water is shed by overlap, with sealant only where the manufacturer's detail calls for it.
Should flashing be replaced with a new roof?
In most cases, yes. Step flashing, pipe boots, kickouts and apron flashing are cheap compared with the roof, and old pieces are often rusted, undersized or installed wrong. Counter flashing on masonry can sometimes be reused when it is in good shape. A reroof is the one time all of it is exposed, so it is the time to get it right.
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